Optical instruments which measure target reflectance and luminescence are employed to indirectly determine diverse target characteristics. For example, one class of instrument measures the light energy emanating from a target batched in direct sunlight at wavelengths (a) within and (b) closely adjacent a selected Fraunhofer Line in the visible or near-visible portion of the spectrum. The results obtained include luminescence information and may be analyzed to classify target spectral characteristics from which mineralogical, geological, and other target characteristics can be deduced, often in conjunction with the analysis of additional target information obtained from measurements taken with other instruments.
In order to achieve the necessary measurement precision, instruments of the above mentioned class must be very closely calibrated, not only in the laboratory, but also in the field under rugged conditions. A typical initial laboratory calibration for an individual instrument requires the use of a standard target illuminated by a standard light source directed under standard conditions onto the standard target from which predetermined readings by the instrument under proof should be obtained. Any deviation from the expected readings may be used to recalibrate the instrument, to derive coefficients involved in the equations for reflectance and luminescence, and/or to develop correction factors to be applied to instrument readings taken in the field. Once a close laboratory calibration has been achieved, slight readjustment to (or slight revision of the correction factors used with) an individual instrument in the field can be performed if reliable field calibration targets are available.
It is possible to achieve effective field calibration of an instrument of this class using one or more calibration targets having known reflectance ratios, known near-Lambertian surface reflectance characteristics (i.e., the radiance being constant or nearly constant for any angle of reflection to the surface normal) and known zero luminescence characteristics. A suitable reflectance calibration target of this class is disclosed in copending U.S. patent application Ser. No. 651023, filed Sept. 17, 1984, entitled REFLECTANCE TEST PLATE HAVING A NEAR-LAMBERTIAN SURFACE AND METHOD FOR MAKING THE SAME, by Joseph E. Zupanick, and assigned to the same assignee as the present invention. However, those skilled in the art will appreciate that both the accuracy and the eae of field calibration would be substantially improved by the employment of field usable calibration plates having both known near-Lambertian reflectance characteristics and known luminescence characteristics which are other than zero in order to obtain a plurality of calibration points on an individual calibration curve.